Reduced clinical incidences of hypotension and improvements in long-term peripheral vascular tone of ESRD patients have been attributed to hemofiltration therapy which is being used in the absence of detailed performance characteristic under clinical conditions. Most available information deals with the effects of blood flow rate on ultrafiltration rate; the more important nephrologically variable solute clearance has not been thoroughly studied. Clearance in hemofiltration depends not only on the ultrafiltration rate of plasma water but on the rejection of solutes by the protein retentative membrane. Rejection data for solutes from blood and plasma are limited, and no systematic study has been made to determine how rejection is influenced by hemofilter operating variables. The objective of this research is to develop experimental test methods and rules to predict the transfer of fluid and solutes during the filtration of blood or plasma through semipermeable hollow fiber membranes. Hollow-fiber filters used in treatment of end stage renal disease (ESRD) will be used as test devices to determine how solute rejection and volume fluxes respond to variables of the device and of use. By using specifically selected test solutes in saline, serum, and whole blood, we expect to test various hypothesis on boundary layer phenomena in order to correlate device geometry, fluid flow conditions, and solution variables with fluxes of solutes and fluids. An ultimate objective of this research, therefore, is prediction of solute clearance under clinical conditions when such filters are used as hemofilters. Such predictions will aid in the design and future development of devices for the separation of plasma components.